This invention relates to a light beam scanning device in which the scanning speed irregularity of a light beam on a scanning surface is corrected and the scanning timing can be finely adjusted.
There are a variety of light beam scanning devices in which a light beam is deflected, so that data is recorded on a recording material placed on the scanning plane of the light beam, or data is read out of an original placed on the scanning plane. The light beam deflectors of these devices are, in general, vibrating mirrors such as galvanometers, or rotary multi-mirrors. In such devices, it is desirable that the light spot on the scanning surface is scanned at a uniform speed. If the scanning speed is irregular, the recorded pattern or the pattern which is obtained by reproducing the data signals read out is distorted. For instance, in the case where the galvanometer is used, the scanning speed is considerably irregular when the accuracy of forming a sawtooth drive signal and the response accuracy or repetition accuracy of the galvanometer are taken into consideration. The irregularity is, in general, increased as the speed is increased. In the case where a rotary multimirror is used, it is very difficult to rotate it at a constant speed. Accordingly, the light spot scanning speed is liable to be unstable.
The above-described irregularity or instability of the optical spot may be substantially eliminated by a known method in which a grid pattern comprising transparent portions and opaque portions arranged alternately with a predetermined period and in a canning direction is placed on a plane which is substantially equivalent to the scanning surface on which data is recorded or read. A photo-electric signal obtained through the grid pattern is employed as a scanning reference signal. FIGS. 1, 2 and 3 show examples of a light beam scanning device employing this method. In these figures, reference numeral 1 designates an optical deflector; 2, a scanning lens; 3, a scanning plane on which a recording material or an original is placed; 5, a grid pattern on a reticle; 6, a condenser lens; 7, a photo-detector; 8, a first laser beam (indicated by the solid line) for scanning a recording material or an original; and 9, a second laser beam (indicated by the broken line) for scanning the grid pattern.
In the device of FIG. 1, the first and second laser beams 8 and 9 are substantially coaxially applied to the deflector 1. These laser beams are deflected by the deflector 1, passed through the scanning lens 2, and separated by a beam splitting mirror 4. As a result, the first laser beam scans the scanning plane 3 on which a recording material or original is placed, while the second laser beam scans the grid pattern 5 on the reticle.
In the device of FIG. 2, the first and second laser beams 8 and 9 are applied to the optical deflector 1 at different angles. In this case, the two laser beams are applied from both sides of the vibrating mirror. The first laser beam 8 is deflected by the vibrating mirror 1 and passes through the first scanning lens 2, to scan the scanning plane 3 on which a recording material or original is disposed. the second laser beam 9 is deflected by the rear surface of the vibrating mirror 1 and passes through the second scanning lens 2', to scan the grid pattern 5.
The device shown in FIG. 3 is excellent in performance and preferable in terms of manufacturing cost. The first laser beam 8 is deflected by the optical deflector 1 which is a vibrating mirror and passes through the scanning lens 2 to scan the scanning plane 3 on which a recording material or original is placed. The second laser beam 9 is applied through a focusing lens 10 to the rear surface of the vibrating mirror to be deflected. Hence, scanning the grid pattern 5 is arranged in the deflecting plane along a circumference with the beam deflecting point as its center. The focusing lens 10 operates to focus the laser beam 9 on the grid pattern 5.
In each of the methods described with reference to FIGS. 1, 2 and 3, the laser beam modulated by the grid pattern is concentrated on the optical detector 7 by the condenser lens 6, so that the scanning is timed with the resultant photo-electric signal as a reference signal. In general, the scanning lens 2 is an f.theta. lens. In this case, the grid pattern 5 is made up of transparent and opaque portions which are alternately provided with a predetermined period.
In the above-described conventional light beam scanning devices, the period of the grid pattern is fixed, and accordingly, only one photo-electric signal is obtained as the scanning reference signal. This is disadvantageous in the case where it is necessary to finely adjust the scanning timing. A typical example where it is necessary to finely control the scanning time is the case where the light beam scanning is applied to a laser printer or laser COM as in this invention. For instance, with the laser printer, a scanning laser beam is used to print the output data of an electronic computer on a sheet on which a form including lists and frames has been printed, or to print the data on a blank sheet together with a form which is exposed to light through a separate optical system.
In this case, the laser beam scanning timing should be determined so that the size of the data is in agreement with the form whose size is predetermined. The accuracy required in this case is very high, 1/3,000.div.0.03% where the number of points to be resolved on the scanning line is 3,000 and the allowable error is one (1) in the number of resolution points. In order that the conventional art realizes this high accuracy, the focal length of the f.theta. lens, the period of the grid pattern and the adjustment of the entire optical system must be extremely high in accuracy. This increases the manufacturing cost of the device.